De la misma forma que en el caso de otros componentes, existen ciertas aplicaciones en las que necesitamos cambiar la capacitancia presentada por un componente en un circuito durante su funcionamiento o para efectos de ajuste.

Un ejemplo está en los receptores de radio en que variamos la capacitancia de un componente para cambiar su frecuencia de operación posibilitando así la sintonía de las diversas estaciones. Los capacitores que pueden tener su capacidad cambiante se denominan capacitores variables y se pueden encontrar en dos tipos básicos: el trimmer que es un capacitor de ajuste y el capacitor variable que es un capacitor de sintonía.

En la figura 1 mostramos el símbolo usado para representar los capacitores variables y los aspectos de los principales tipos encontrados en los equipos electrónicos.


| Haga click en la imagen para ampliar |

 

 

 

En (a) tenemos un capacitor variable de tipo antiguo encontrado en radios y sintonizadores. En (b) tenemos un capacitor variable de radios modernos y en (c) algunos tipos de trimmers comunes.

 

Especificaciones:

a) Capacitancia

Los capacitores variables y trimmers pueden ser especificados por la capacitancia máxima que presentan como por el rango de capacitancias que pueden asumir. Así, un trimmer de 3-30 pF es un trimmer que puede ser ajustado para tener capacidades entre 3 pF y 30 pF. La capacitancia mayor se obtiene cuando el componente está totalmente "cerrado", o sea, la mayor área de las armaduras se enfrenta.

 

b) Tipo

El tipo de capacitor variable o trimmer se especifica por el número de secciones y el material utilizado como dieléctrico. Los capacitores antiguos utilizan como dieléctrico el propio aire (no a ningún material separando las armaduras) mientras que los tipos más modernos utilizan plásticos.

 

c) Tensión

En muchas aplicaciones es importante saber cuál es la tensión máxima que podemos aplicar entre las armaduras del capacitor sin que ocurra el faiscamiento o el rompimiento del material del dieléctrico. En particular, esta especificación es importante en transmisores donde las tensiones de hasta miles de voltios pueden aparecer entre las armaduras de un capacitor.

 

Donde se encuentran:

Los capacitores variables se utilizan principalmente en circuitos de alta frecuencia tales como receptores de radio, telecomunicaciones, televisores, transmisores y en muchos otros donde las señales de frecuencias superiores a 100 kHz están presentes y necesitan ser ajustadas.

Hoy en día, los capacitores variables son componentes algo raros y los trimmers sólo se pueden obtener en valores muy restringidos. Sin embargo, para los montajes, cuando tales componentes no se pueden encontrar en los proveedores, siempre existe la posibilidad de encontrar uno en una radio u otro equipo fuera de uso.

Un tipo de capacitor variable ajustable poco encontrado en nuestros días es el "padder" cuyo aspecto se muestra en la figura 2.

 

 

Figura 2 - Padder de porcelana - foto Rey del sonido
Figura 2 - Padder de porcelana - foto Rey del sonido

 

 

Este capacitor está formado por varias placas que, al ser apretadas por un tornillo, hacen que la capacitancia aumente.

 

 

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